Simultaneous infiltrating and obtaining a brazable surface



improved and more economical nite States atent 2,851,381 Patented Sept.9, 1958 SIIVIULTANEOUS INFILTRATING AND OBTAIN- ING A BRAZABLE SURFACENorman S. Hoyer, Pittsburgh, Pa., assignor to Gibson Electnc Company,Pittsburgh, Pa., a corporation of Pennsylvania No Drawing. ApplicationApril 5, 1955 Serial No. 499,517

3 Claims. (Cl. 117-227) My invention relates to improvements ininfiltrated type powdered metals and more particularly to an improvedmethod for manufacturing electrical contacts. The improvement consistsof preparing one surface of such a contact so that it is more easily andreadily welded or silver soldered to its supporting arm.

The conventional steps in one method of manufacture of electricalcontact materials comprising a refractory metal such as tungsten,molybdenum, or their carbides and a lower melting point conducting metalsuch as silver are as follows:

The refractory powders are either pressed alone or mixed with acomparatively small percentage of silver before pressing. The pressed orgreen compact is then sintered in a furnace using a hydrogen or otherreducing atmosphere. The sintering temperature may be either above orbelow the melting point of the lower melting point constituent. Aftersintering, porosity is calculated and from this weight of silver orcopper (whichever the case may be) that is to be infiltrated. One of themore common methods of infiltrating a porous compact is to place theinfiltrant on top of the compact, the compact being generally supportedby a graphite boat. The infiltrating temperature is somewhat above themelting point of the infiltrant. To be certain of complete infiltrationan amount of infiltrant in excess of that which can be absorbed by thecompact, is used. Thus, a small amount of excess infiltrant generallyremains in the form of a lump or mound on the surface of the infiltratedcompact. Such residual infiltrant serves no useful purpose and, theusual procedure is to remove this lump and then solder-flush the surfaceof the contact which is to be welded or silversoldered to a contactcarrying arm. I have found that by applying a small amount of a finepowder from the group consisting of iron, cobalt or nickel to theinfiltrating surface of the sintered compact prior to the infiltrationstep, the subsequent solder-flushing step may be eliminated. In anyevent, the excess infiltrant is spread over the surface of the compact,and the more uniform the percentage of is determined the coating ofinfiltrant which results provides a surface which is more readily weldedor silver-soldered to the contact arm of the switch, circuit breaker orother electrical device to which the contact is to be secured.

Accordingly, it is an object of my invention to provide a method forproducing a refractory-base material having a surface which is morereadily welded or silversoldered.

' It is another object of my invention to provide an method ofmanufacturing electrical contacts which are readily welded or silversoldered to a contact carrying means and which comprises a pressed andsintered refractory metal and a lower melting point conducting metal.

'These and other objects of my invention will become more apparent fromthe following description.

Essentially my invention involves, in the infiltration of the compactedrefractory, the application, to the infiltrat ing surface thereof, of asmall amount of a powdered metal which in effect acts as a wetting agentfor the surface of the compact and allows excess infiltrant to spreadevenly over the surface.

Such a Wetting agent may alleviate or even eliminate the need forapplying a solder-flush or other coating to the external surface of theinfiltrated compact in order to make it readily weldable or more easilysilver-soldered to another metallic surface. Even if a solder-flush isrequired, it may eliminate the necessity of removing excess silverbefore solder-flushing. In any event, it has a marked tendency to causethe infiltrant to spread evenly over the infiltrating surface withoutleaving unsilvered areas on the said surface. This is particularlyimportant in the case of electrical contacts wherein it is necessary toobtain a secure joint between the contact and the contact carrying armof the particular switch or circuit breaker to which the contact isattached.

By applying a finely divided powder of iron, cobalt, or nickel to thesurface of the compact to which the infiltrant is to be applied prior tothe infiltration step, the excess infiltrant is spread evenly over thesurface of the refractory and in itself provides a surface which may bereadily welded or silver-soldered when the contacts are attached to ametallic member.

The powder which I used in my invention is of approximately 200 meshfineness but may vary generally between and 300 mesh. Although I preferto use nickel, cobalt and iron may also be employed.

The use of this finely divided powder prior to applying the infiltrantto the pressed and sintered compact may eliminate the necessity of usingserrations or corrugations on the surface of the contact for the purposeof spreading the excess infiltrant. This is advantageous, particularlyin those instances where serrated contacts are not acceptable. It shouldbe noted, however, that my invention may also be used in connection withserrated contacts, if desired.

The electrical contacts to which I refer, generally comprise tungsten,molybdenum, tungsten carbide, molybdenum carbide or mixtures thereof.The infiltrating material is normally either copper or silver, generallypresent in amounts of twenty to sixty percent.

As hereinbefore stated it has been found that copper or silver typeelectrical contacts braze or weld more efifectively to another metallicmember (such as a contact carrying arm of a circuit breaker) either whenthe surface has been previously solder-flushed or a coat of silver isotherwise applied thereto. The necessity for such a coating operation iseliminated by virtue of the formation of the uniform layer of the excessinfiltrant resulting from the use of my novel wetting agents. Moreover,my wetting agent also assures uniform overall thickness of the contactprovided that the floor of the furnace is fiat at the time ofinfiltration.

The nickel or other finely divided powder is spread over the surface ofthe compact to be infiltrated in any suitable fashion, such as by theuse of a small artists type brush. The nickel powder may be mixed withCCL; or other volatile vehicles so that the application is made morequickly and easily. This allows generally for a more uniformdistribution of the nickel. The vehicle volatizes quickly and does notinterfere in any way with the subsequent infiltration. A sprayingtechnique may be used if desired.

The amount of powder used for any given compact generally is one percentof the weight of the infiltrant, but may vary generally between 0.2 and2 percent. The powder does not impede the infiltrant from permeatinginto the interconnected pores of the compact. It causes a wetting of therefractory surface which makes the infiltrant, silver or copper, spreadmore easily and cover that inch. The pressed compacts are 'ducingatmosphere maintained Specific example A mixture of silver and tungstenpowders containing about grams of tungsten and 5 grams of silver areintimately mixed. The powders are then pressed in powder metal pressesusing a pressure of tons per square then sintered in a reat 2000 F. forabout minutes. The sintered compact is then infiltrated with silver,using about 6 grams of silver, at about 2000 F. This is suflicient toinfiltrate the pores of the compact.

Prior to the infiltration, .06 gram of a 200 mesh nickel powder isbrushed on the surface of the sintered compact. The temperature is thenraised to 2200 F. for fifteen minutes and the silver infiltrates intothe compact, the surface thereof having an even layer of excess silver.

The same procedure, when followed without the addition of the nickelpowder to the infiltrated surface produces a surface having a localizedlump of excess silver.

In the foregoing, I have described my invention only in connection withpreferred embodiments thereof. Many variations and modifications of theprinciples of my invention within the scope described herein areobvious.

Accordingly, I prefer to be bound not by the specific 4 disclosureherein, but only by the appending claims.

I claim:

1. In the manufacture of an electrical contact comprising a pressed andsintered refractory compact from the group consisting of tungsten,molybdenum and carbides thereof and infiltrated with from 20 to percent,by weight, of a lower melting point conductive metal from the groupconsisting of copper and silver, the improvement which comprisesapplying to only the infiltrating surface of the pressed and sinteredrefractory compact 0.2 to 2 percent by weight of a finely dividedmetallic wetting agent from the group consisting of iron, cobalt andnickel, and subsequently impregnating the said compact through the saidcoated infiltrating surface with the said lower.

melting point conductive metal, thereby causing the excess infiltrant tospread evenly over-the said surface.

2. The method as set forth in claim 1 in which the metallic wettingagent is in the form of a powder of from to 300 mesh particle size.

3. The method of claim 2 in which the powder is nickel.

References Cited in the file of this patent UNITED STATES PATENTS

1. IN THE MANUFACTURE OF AN ELECTRICAL CONTACT COMPRISING A PRESSED ANDSINTERED REFACTORY COMPACT FROM THE GROUP CONSISTING OF TUNGEN,MOLYBDENUM AND CARBIDES THEREOF, AND INFILTRATED WITH FROM 20 TO 60PERCENT, BY WEIGHT, OF A LOWER MELTING POINT CONDUCTIVE METAL FROM THEGROUP CONSISTING OF COPPER AND SILVER, THE IMPROVEMENT WHICH COMPRISESAPPYING TO ONLY THE INFILTRATING SURFACE OF THE PRESSED AND SINTEREDREFRACTORY COMPACT 0.2 TO 2 PERCENT BY WEIGHT OF A FINELY DIVIDEDMETALLIC WETTING AGENT FROM THE GROUP CONSISING OF IRON, COBALT ANDNICKEL, AND SUBSEQUENTLY IMPREGNATING THE SAID COMPACT THROUGH THE SAIDCOATED INFILTRATING SURFACE WITH THE SAID LOWER MELTING POINT CONDUCTIVEMETAL, THEREBY CAUSING THE EXCESS INFILTRANT TO SPREAD EVENLY OVER THESAID SURFACE.